Flash photography has been widely used for decades. It is a technique whereby a scene is illuminated by an intense pulse of light, generated by a camera flash, as the camera records the image. A few years ago the most popular type of camera flash was a xenon flash tube. These were fitted into most consumer cameras. With the advent of digital photography cameras have been made smaller, and this has created a strong demand for smaller flash units, especially on mobile camera phones. Recently advances in LED (light emitting diodes) technology have been made such that modern LEDs can produce pulses of light that are intense enough to be used as a camera flash, and in addition an LED camera flash has potential to be made smaller than a xenon flash tube.
A disadvantage of LEDs is that the pulse of light from an LED is emitted from a very small region. If viewed inappropriately, this light could be focused to a small and therefore intense spot on a person's retina. Therefore in the interest of safety the intensity of a directly viewed LED would have to be reduced. An example of a basic low intensity LED flash design is given by U.S. Pat. No. 7,136,672 (published 14 Nov. 2006).
The current state of the art in camera phone flashes use LEDs in conjunction with additional optical elements. In this flash the light from one or more LEDs 1 is spread out with various optical elements, for example a prism film 2 and a Fresnel lens 3, so that the light is emitted from a relatively large region of the phone. In this way the flash can be bright and safe. A sketch of the design is shown in FIG. 1.
This design is not ideal, as it is thick and some light is lost as it passes through the system.
A different type of flash is known in the field of professional photography. Camera flashes (which illuminate from a point) can produce unnatural images with strong shadows and specular reflections from skin which can makes faces look too shiny. Professional photographic studios would use very diffuse lighting. Some professional photographers use large ‘ring flashes’ which illuminate the subject from a wider range of angles to give a better picture. FIGS. 2(a) and 2(b) are a front view and a side view of a camera having a ring flash 4 attached to the camera body 5 such that the ring flash 4 is approximately concentric with the camera lens 6. An example of this type of flash is sold by the company Olympus (model number SRF-11 Ring Flash Set).
Most ring flashes are rings of xenon flash tube. An adapter for xenon flashes has been designed to attach to a regular xenon flash, and reflect the light around a ring so that it is emitted in a ring. This type of flash is called the ‘Coco ring flash adapter’ and is available from, for example, Microglobe Photo Equipments Limited, London, UK. These types of ring flash are large and not suitable for miniaturisation.